Complete automation of molded case circuit breaker components by robotic assembly has not heretofore been completely successful. One impediment to complete robotic assembly is the attachment of a flexible conductive braid between the circuit breaker contact and the circuit breaker load terminal lug.
Early attempts to eliminate the electrical contact braid are found in U.S. Pat. Nos. 3,023,292, 3,033,964 and 3,073,936 wherein a pair of contact arms are supported on a contact arm support by means of a pivot pin and a thick spring clip is fastened to the contact arm support and arranged around both the contact arms and the support. Direct electrical connection between the spring clip and the terminal conductor in some industrial-rated circuit breaker designs advantageously improves the electrical conduction between the terminal conductor and the movable contact arm by electromagnetic forces of attraction generated by the current through the spring clip. The increasing electric current increases the electromagnetic force on the juncture between the movable contact arm and the terminal conductor to create an increasing compressive force therebetween.
In lower ampere-rated current-limiting industrial circuit breakers, the forces exerted by the spring clip on the movable contact arm and the terminal conductor must remain relatively constant with increasing current to ensure that the contacts can be electrodynamically repulsed and separated under high current faults such as those occurring with short circuits. The contact arm must rapidly move about its pivot in the early stages of the current wave-form to separate the contacts with minimum let-through current at the instant of separation. This is not easily obtained when the compressive forces on the movable contact arm and the terminal conductor substantially increase at the time the movable contact arm is required to rotate about its pivot.
A more recent design that enables a braidless movable contact arm that is robotically assembled is described in U.S. Pat. No. 4,733,033. This patent discloses the use of a spring having a planar configuration capable of holding the contact arm against its support posts with sufficient force to maintain electrical contact during overcurrent conditions. When this design is used within higher ampere-rated current limiting industrial circuit breakers, a parallel current path should be connected between the movable contact arms and the contact arm support posts to prevent the occurrence of arcing between the contact arm and the support posts under intense short-circuit overcurrent conditions.
U.S. Pat. No. 4,931,603 describes the use of bifurcated shunt plates to reduce the occurrence of arcing between the movable contact arm and the support posts. U.S. patent application Ser. No. 764,287 filed Sep. 23, 1991 describes the addition of a flexible braid conductor of reduced diameter in combination with a U-shaped spring in lieu of the bifurcated shunt plates, as a cost improvement. To use the smaller braid conductor, an offset tab is welded between the end of the braid conductor and the contact arm support plate to prevent the braid from becoming frayed upon long term subjection to flexing under high current conditions.
In a further attempt at cost reducing the movable contact arm assembly, it was discovered that the proposed contact arm assembly could be used over a wide range of ampere ratings without requiring a separate assembly for each increasing ampere rating.
One purpose of the instant invention accordingly is to describe a movable contact arm assembly that includes a parallel current path between a single movable contact arm support leg and a pair of flexible braid conductors attached to the movable contact arm support.